1. Field of the Invention
The invention relates to an oriented, heat-sealable, multilayer polyolefin film comprising a base layer containing polypropylene and resin, and at least one heat-sealable outer layer. The films are distinguished by excellent heat-sealing properties and excellent running properties on high-speed packaging machines.
2. Description of the Related Art
The demands made of the processing properties of films and their smooth passage through automatic machines have increased continually over the years. Modern automatic packaging machines as used, for example, in the cigarette industry, operate at a rate of 350 to 450 wrappers per minute. Films which run on such machines require properties which are carefully balanced with respect to each other. The problem that frequently occurs is that optimization of one parameter unacceptably impairs another property.
Films which have good running properties due to a balanced synergistic formulation are known. EP-A0,468,333 describes a multilayer polypropylene film having improved barrier properties whose base layer contains a hydrocarbon resin having a softening point of greater than or equal to 140.degree. C. The multilayer film has an outer layer of propylene copolymers or terpolymers. The base layer and/or the outer layer contain(s) an antiblocking agent and/or lubricant. These films have unsatisfactory blocking properties at elevated temperatures.
EP-A-0,182,463 describes a multilayer heat-sealable film containing 0.05 to 0.2% by weight of tertiary aliphatic amine in the base layer and a combination of silicone oil and SiO.sub.2 in the heat-sealable outer layer.
According to the description, the surprising interaction of SiO.sub.2, silicone oil and amine in combination with a selected outer layer thickness of less than 0.8 .mu.m gives films having a coefficient of friction of 0.3 or less. This film has the disadvantage that the outer layer thickness cannot exceed 0.8 .mu.m, without greatly impairing the running properties.
EP-A-0,143,130 discloses multilayer heat-sealable films containing a carboxamide in the base layer and also a combination of silicone oil and SiO.sub.2 in the outer layer. As in EP-A-0,182,463 mentioned above, a synergistic effect of the three selected components on the coefficient of friction is described. However, these films have deficient blocking behavior at elevated temperature, in spite of their advantageous surface slip.
EP-A-0,194,588 and EP-A-0,217,598 describe films in which silicone oil, and if desired in combination with SiO.sub.2, is incorporated only into one outer layer. The second, silicone oil-free outer layer is corona-treated to improve the printability. The transfer of silicone oil onto the surface of this treated second outer layer subsequently takes place by contact with the first, silicone oil-containing outer layer. This feature gives a silicone oil-treated film having good surface-slip characteristics which is simultaneously readily printable on the corona-treated side, but is nevertheless heat-sealable.
The films of the prior art are all unsatisfactory with respect to their running properties, their heat-sealing properties and their blocking values at elevated temperature. There is a continuous need in the packaging industry to keep packing costs as low as possible. For this reason, it is always attempted to allow the machines to run at minimum speed, since this is associated with greater economic efficiency. An important limiting factor for the maximum speed is the film with its properties.